The present invention relates to the registration of color images in a digital color printer and, more particularly, to an improved method and apparatus for detecting lateral registration errors between color separations in Raster Output Scanner (ROS) systems.
In recent years, digital color printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer uses a plurality of imagers, either ROS or LED print bars, to expose the charged portions of a photoconductive member to record an electrostatic latent image thereon. Generally, a ROS incorporates a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is incident on a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from each facet and thereafter focused to a spot on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium with a plurality of scan lines in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream, at a rate such that individual picture elements ("pixels" ) of the image represented by the data stream are exposed on the photoconductive member to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
Color digital printers may operate in either a single pass or multiple pass mode. In a single pass, process color system, three ROS imagers are positioned adjacent to a moving photoreceptor surface and are selectively energized to create successive image exposures, one for each of the three process colors, cyan, magenta and yellow. A fourth black imager is usually added. A color digital printer may also operate in a highlight color mode wherein one or two colors and black are exposed.
In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scan lines formed by the modulated beam generated by the imagers. With either system, each image is typically formed within a tight tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction and in the direction perpendicular to the process direction (referred to as fast scan, lateral or transverse registration). The description "lateral" will be used hereafter.
In the past, the problems associated with lateral registration of pixels in multiple exposures of images on photoreceptive belts have been addressed either by attempts to guide the belt in a manner to avoid or at least reduce lateral belt shifting, or by corrective steering of the belt in response to sensed lateral deviation from true linear travel. Belt steering techniques previously employed have most commonly relied on belt edge sensors to detect lateral shifting of the belt and to control a mechanical steering mechanism for returning it to the correct line of travel. Such belt steering systems are subject to inaccuracy resulting from belt width tolerances and edge waviness and also require a mechanical steering mechanism which requires a substantial measure of belt travel and time before the electrically sensed belt shifting can be accomplished. Registration in the transverse direction of ROS systems is known in the prior art and registration techniques are disclosed in copending applications, U.S. Ser. No. 07/635,835 filed on Jan. 3, 1991, U.S. Ser. No. 07/807,927 filed on Dec. 16, 1991, and U.S. Ser. No. 07/821,526, filed on Jan. 16,1992, all assigned to the same assignee as the present invention.
The following patents and applications may be relevant to various aspects of this invention.
U.S. Pat. No. 4,912,491 to Hoshino discloses an apparatus for forming superimposed images as well as forming registration marks corresponding to the position of the images associated therewith. The registration marks are formed apart from the imaging portion of the medium and are formed in a transparent area so as to be illuminable from below. Thus, detectors detect the position of the registration marks as the marks pass between the illuminated areas. The detection/sensing of marked positions is used in determining proper registration positioning whereby the image forming devices may be adjusted to achieve such registration.
U.S. Pat. No. Re. 32,967 to St. John et al., a reissue of U.S. Pat. No. 4,485,982, issued Dec. 4, 1984, discloses a web tracking system for a continuous web which passes along a predetermined path through one or more processing stations. The tracking system has aligned tracking indicia on one or both sides of the web and means are provided for sensing these indicia which are indicative of dimensional changes in width and length of the web at a particular point and further there is also provided an edge sensor to determine movement of the web itself.
U.S. Pat. No. 4,961,089 to Jamzadeh discloses an electrostatic reproduction apparatus having a web tracking system wherein the web rotates about image processing stations with a plurality of rollers. A guide means is provided to move the web around the rollers. The guide means include a steering roller which is actuated according to a web tracking system.
The teachings of the above-referenced disclosures, however, have not relieved the need for a relatively low cost, simple and accurate registration system. That is, a system permitting superimposing successive images formed on a surface moving relative to the imaging stations by compensating for deviations in the travel of the belt from a predetermined aligned condition.